Self-anti-reflective density-modulated thin films by HIPS technique

Abstract

A critical factor for an efficient light harvesting device is reduced reflectance in order to achieve high optical absorptance. In this regard, refractive index engineering becomes important to minimize reflectance. In this study, a new fabrication approach to obtain density-modulated CuIn GaSe (CIGS) thin films with self-anti-reflective properties has been demonstrated. Density-modulated CIGS samples were fabricated by utilizing high pressure sputtering (HIPS) at Ar gas pressure of 2.75 × 10 mbar along with conventional low pressure sputtering (LPS) at Ar gas pressure of 3.0 × 10 mbar. LPS produces conventional high density thin films while HIPS produces low density thin films with approximate porosities of ∼15% due to a shadowing effect originating from the wide-spread angular atomic of HIPS. Higher pressure conditions lower the film density, which also leads to lower refractive index values. Density-modulated films that incorporate a HIPS layer at the side from which light enters demonstrate lower reflectance thus higher absorptance compared to conventional LPS films, although there is not any significant morphological difference between them. This result can be attributed to the self-anti-reflective property of the density-modulated samples, which was confirmed by the reduced refractive index calculated for HIPS layer via an envelope method. Therefore, HIPS, a simple and scalable approach, can provide enhanced optical absorptance in thin film materials and eliminate the need for conventional light trapping methods such as anti-reflective coatings of different materials or surface texturing.

Influence of The Substrate-Target Angle and Sputter Temperature On The Properties of CIGS Thin Films Sputtered From Single Quaternary Target

“…Columnar structure formation is apparent in thin films deposited at higher angle of 60 0 (Figure 1d-f) while relatively more planar and smooth thin films are formed in conventional configuration with an angle of 40 0 (Figure 2a-c). The main reason behind the difference in the microstructure formation is the shadowing effect [22] that becomes significant when the arriving atoms reach the substrate at higher oblique angles. In our case, the shadowing effect which results in columnar formation is observed at the angle of 60 0 since the target atoms have a more oblique trajectory towards the substrate.…”

“…Thus, more molecules interact, and the grain size is enhanced which results in the widening bandgap [37]. The slight enhancement in bandgap for the films sputtered under the incident angle of 60 0 compared to their counterparts can be attributed to the morphological and elemental differences which may result from their material densities [22].…”

“…On the other hand, the impinging atoms tend to form a porous film with columnar microstructure at highly oblique angles due to the cluster formation at the beginning of the deposition. When the angle of incidence is high, the incoming target atoms pile up and form randomly assembled islets on the film surface due to the phenomenon called the "shadowing effect" [21][22][23] which can be explained as the formerly clusters' shadowing the later atoms. These islets continue to grow and join each other during the sputtering process, and thus a porous, thin film that has a columnar type of microstructure is formed eventually.…”